The invention relates to a steam generator, which can be used, for example, in a nuclear installation, for the production of electricity or for naval propulsion.
A steam generator of conventional type comprises a cylindrical outer vessel with a vertical axis, resistant to pressure and closed at each end by a dome. A horizontal tubular plate is arranged inside the vessel, fast to the latter and defining with the lower dome a chamber separated into two collectors, for the intake and discharge, respectively, of a primary heat transfer fluid. A bundle of U-shaped tubes is mounted on the tubular plate. Each U-tube has a hot limb which communicates with the intake manifold and a cold limb which communicates with the discharge manifold, the group of hot limbs constituting the hot branch and the group of cold limbs constituting the cold branch. A secondary envelope surrounds the bundle without being supported on the tubular plate and defines an annular space with the outer vessel. A water intake device is provided for supplying the annular space with secondary water. This secondary water reascends along the tubes of the bundle and is vaporized in contact therewith. In the upper part of the vessel is a set of separators; the vapor produced is removed through an opening formed in the upper dome. The water flowing from the separator, called recycled water, returns to the tubular plate by using the annular space comprised between the outer vessel and the secondary envelope, before reascending again along the bundle of U-shaped tubes. With this recycled water is mixed a feed water which comes from the intake device referred to above. This intake device is placed in the upper part of the steam generator in order that the mixture of feed water and recycled water may be homogeneous before its entry into the tubular bundle.
The conventional type of generator that has just been described has the drawback that the primary-secondary exchange surface is not optimally used; consequently, the thermal efficiency of the whole of the generator is not at a maximum.
To overcome this drawback, i.e., to attempt to increase the efficiency of the generator, provision has been made for pre-heating the feed water before mixing it with the recycled water. This pre-heating permits a better use of the exchange surface and an increase in the thermal efficiency of the whole of the generator.
From a theoretical point of view, the maximum efficiency is reached when the feed water is brought into the immediate vicinity of the tubular plate and then reascends along the cold branch of the U-tubes, in contact with which it must be reheated over a certain distance, called the preheating zone. The feed water is only mixed with the recycled water coming from the separators at the outlet from the preheating zone.
However, this arrangement has several drawbacks. In particular, due to the fact that the feed water must not be mixed with the recycled water before having been pre-heated, the intake and guide structures for this water must be relatively fluid-tight. In addition, due to the fact that under low load or during certain transient periods of operation (introduction of emergency water), the temperature of the feed water decreases. This water can then initiate in the structures with which it comes into contact (tubes, tubular plate, pressure vessel) unacceptable thermal stresses.
To overcome these drawbacks, several solutions have been proposed, among which are those which consist of incorporating a support in temperature of the feed water or of providing a secondary feed at the top of the steam generator.
These two solutions, however, lead to encumbrance of the operation and of the control of the steam generator.
To reduce the heat stresses in the tubular plate, the zone where the feed water enters the bundle may be moved away from this plate while keeping it in the bottom part. This is the subject, for example, of French Pat. No. 2,191,704 in the name of SIEMENS A. G.
This geometry manifests itself with respect to the theoretical case envisaged above by a lowering in the saturation pressure delivered by the steam generator and by complex structures to be installed in the lower part of the apparatus.
It has also been proposed to create a buffer zone traversed by the recycled water above the tubular plate.
It would be possible, for example, to refer to French patent application No. 2,285,573 in the name of WESTINGHOUSE ELECTRIC CORPORATION. In the buffer zone, the tubular plate, on the cold branch side, is swept by the recycled water. This recycled water is guided by a very slightly permeable plate, which is in fact constituted by the floor of the pre-heater, this pre-heater and the feed water intake device being situated at the lower part of the steam generator.
However, the floor of the pre-heater may create bending stresses in the tubes (pressure deformation of the tubular plate, differential expansion of the tubular plate and of the floor of the pre-heater), and in addition renders the tubing operation difficult on account of its having a lower permeability than the cross-bracing plates of the tubular bundle, and of its proximity to the tubular plate.
On the other hand, this floor cannot be perfectly fluid-tight, so that a portion of the feed water passes through it and is directed to the hot branch: this reduces the overall performance of the steam generator.
In addition, the introduction of feed water into the bundle without mixing with recycled water, whatever the load, makes delicate the establishment of the sizes of the pre-heater structures since the latter must respond to two contradictory requirements:
1. It must accommodate considerable variations in temperature, which necessitates small thicknesses.
2. It must limit in an accident situation (rupture of tubing, water-hammer) deformations which could place in danger the primary-secondary barrier constituted by the tubes. It is hence necessary to provide a rigid assembly.
Finally, the steam generator of the type described in the French patent application No. 2,285,573 has the drawback of comprising a more restricted space for the tubes for a given diameter of the pressurized envelope, a part of the space being occupied by the pre-heater.
French patent application No. 2,387,417 in the name of the Commissariat a l'Energie Atomique is also known, this describing a steam generator not having the aforementioned drawbacks. In this generator, all of the feed water is mixed with the recycled water and sent to the cold branch of the tubular plate through a central caisson situated between the limbs of the U-tubes, the water being channelled to the caisson by two flat rings of elliptical shape.
However, this construction leads to a very complex structure.
It is an object of the invention, therefore, to provide a novel steam generator provided with a pre-heating device but not having the drawbacks of the prior art.